1. Field of Invention
The present invention relates to the field of marine seismic data acquisition systems and methods of using same. More specifically, the invention relates to systems and methods for positioning of seismic streamers.
2. Related Art
Marine seismic exploration investigates and maps the structure and character of subsurface geological formations underlying a body of water. For large survey areas, a seismic spread may have vessels towing multiple seismic streamer cables through the water, and one or more seismic sources by the same or different vessel. The seismic sources typically comprise compressed air guns for generating acoustic pulses in the water. The energy from these pulses propagates downwardly into the geological formations and is reflected upwardly from the interfaces between subsurface geological formations. The reflected energy is sensed with hydrophones attached to the seismic streamers, and data representing such energy is recorded and processed to provide information about the underlying geological features.
Streamers are typically positioned with acoustic ranging devices attached to or integrated into the streamers. The performance of the acoustic ranging devices varies throughout the spread for a variety of reasons. These hindrances to acoustic propagation include bubbles in the water column generated by the seismic source, density layering in the water column causing refraction and reflection of the acoustic energy, and interference from bottom reflected signals. In an attempt to deal with the degraded acoustic performance due to these factors, the individual distance measurements are often manually and interactively filtered before the distances are introduced to the position estimation algorithms. Further, the subjectivity of manual filtering can give position estimate biases, and is in any case labor intensive. In some cases the degraded acoustic performance is so severe that filtering is not possible and re-acquisition is required, (at great expense) and only possible when the acoustic propagation environment changes to allow it. When adequate relative positions are determined through acoustic ranging and other types of relative positioning measures such as compasses, baseline distances, and inertial positioning units, a link is needed to an earth fixed reference frame. This is typically achieved through the Global Positioning System (GPS) using receivers mounted on buoys or floats attached to the streamers and sources of the seismic spread. These floats are connected at points away from the seismic hydrophones to prevent tugging noise at the hydrophones. The longer the distance along the streamer away from the GPS control points, the greater the degradation of positioning accuracy throughout the spread. The current state of the art is to asymmetrically deploy tail buoys at the front and tail of the spread, with fewer floats at the front due to the towing and handling challenges encountered near the seismic source.
Steering devices along the streamer cause the streamer to deform in a deterministic way. The physics of the deformation have been described elsewhere. Conventional streamer positioning uses compasses and assumes a straight shape. On a streamer using lateral steering devices, such as steerable birds known under the trade designation Q-FIN™, from WestemGeco L.L.C., Houston, Tex., the streamer shape is non straight due to lateral forces exerted by the wings of the steerable birds. Direction measures tangent to the cable will give erroneous position estimates on a steered cable since the conventional straight cable assumption is untrue. In order to address this problem, intrinsic ranging using modulated acoustics was developed to give a high density of acoustic points along the streamer. The points are so close that an arc formed between them is approximately a straight line. However, these acoustic ranging techniques still require knowledge of the acoustic propagation velocity, which as explained is not constant through a spread.
An alternate method of positioning acoustic receivers along a data acquisition cable or streamer that would not depend on acoustic propagation velocity would be beneficial. The methods and systems of the present invention address this problem.